In recent years, with the spread of portable electronic devices such as notebook personal computers, cell phones and the like, there is a large need for development of compact and lightweight secondary batteries having a high energy density. Further, as a battery for an electric vehicle including a hybrid automobile, the development of a secondary battery having high output characteristics is strongly desired. As a non-aqueous electrolyte secondary battery that satisfies such needs, there is a lithium-ion secondary battery. A lithium-ion secondary battery has an anode, a cathode and an electrolyte, and as the active material of the anode and the cathode, a material is used for which desorption and adsorption of lithium is possible.
Currently, much research and development is being performed for lithium-ion secondary batteries, and particularly, lithium-ion batteries that use a layered or spinel type lithium composite oxide as the cathode active material can obtain a 4V class high voltage, so practical application as a battery having high energy density is advancing.
As the lithium composite oxide that is used as the cathode active material of a lithium-ion secondary battery, currently, lithium cobalt composite oxide (LiCoO2) for which the composition is comparatively simple, lithium nickel composite oxide (LiNiO2) that uses nickel that is less expensive than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1/3Co1/3Mn1/3O2), lithium manganese composite oxide (LiMn2O4) that uses manganese and the like have been proposed.
Of these, lithium nickel composite oxide displays a large charge and discharge capacity, so it is expected to become a cathode active material from which a secondary battery having high energy density can be manufactured. However, pure lithium nickel composite oxide has problems with thermal stability and cycling characteristics in the charging state, and thus use as a practical battery would be very difficult.
Due to such a situation, an attempt was made to improve the thermal stability and cycling characteristics by replacing part of the nickel of particles of a lithium nickel composite oxide with other metal elements. For example, JPH05-242891 (A) discloses a lithium nickel composite oxide that is expressed by the general formula: LiaMbNicCodOe (where M is at least one metal that is selected from among Al, Mn, Sn, In, Fe, V, Cu, Mg, Ti, Zn and Mo, and 0≤a≤1.3, 0.02≤b 0.5, 0.02≤d/c+d≤0.9, and 1.8≤e≤2.2, and b+c+d=1). By replacing part of the nickel of this lithium nickel composite oxide with a metal, particularly, Cu or Fe, it is possible to suppress change in the crystal structure, and to improve the discharge capacity and thermal stability of the secondary battery.
However, normally, unreacted lithium salts such as lithium carbonate, lithium nitrate and the like remain as impurities inside the lithium nickel composite oxide after formation. Therefore, when a secondary battery that uses this kind of lithium nickel composite oxide as a cathode active material is charged under high-temperature conditions, there is a possibility that the unreacted lithium salts will undergo oxidative decomposition. In that case, the generated decomposition gas may cause a problem of improper secondary battery dimensions or a decrease in the battery characteristics.
In the case of such problems, removing the lithium salts by washing the formed lithium nickel composite oxide with water and drying is effective. For example, JP2011-034861 (A), JP2003-017054 (A) and JP2007-273108 (A) disclose manufacturing methods of a lithium nickel composite oxide having a washing process that, by optimizing the amount of water used in washing and the amount of washing time, removes impurities such as lithium carbonate while suppressing the elution of lithium. In these manufacturing methods, after the washing process, it is necessary to perform heat treatment in an air atmosphere, non-carbon atmosphere, or vacuum atmosphere in order to remove the moisture content.
However, when heat treatment is performed in an air atmosphere, the lithium that exists on the surface of the lithium nickel composite oxide reacts with carbon in the air and becomes lithium carbonate, so it is not possible to solve the problem described above. On the other hand, when heat treatment is performed in a non-carbon atmosphere, or in a vacuum atmosphere, the generation of lithium carbonate is suppressed, however, part of the lithium that exists on the surface is proton-exchanged with the hydrogen ion in the washing solution, and results in a state close to an oxynickel oxide (NiOOH) state, so a problem occurs in that the electrical conductivity of the cathode active material that is obtained is impaired. Moreover, due to proton exchange, hydrogen is arranged in the site where the lithium was originally arranged, so there is also a problem in that during charging and discharging, the diffusion of lithium ions is obstructed, and the reaction resistance increases. Furthermore, due to washing with water, the strength of the cathode active material decreases, so a problem occurs in that the cathode active material cracks due to rolling pressure that is applied while forming the electrode, and the particles that are generated due to that become obstacles, so it becomes impossible to obtain a highly dense electrode.
In regard to this, JPH09-231963 (A) and JP2010-155775 (A) disclose technology in which after washing is performed under specified conditions, heat treatment is performed on the lithium nickel composite oxide in an oxygen atmosphere and at a temperature of 400° C. or greater. By performing heat treatment in an oxygen atmosphere in this way, it becomes possible to stabilize the crystallinity of the particle surfaces.
However, in the technology of the literature above, it is necessary to perform the washing of the lithium nickel composite oxide using pure water having mass that is two times or more than the mass of the lithium nickel composite oxide. Therefore, there is a possibility that a problem will occur in which the slurry concentration during washing becomes excessively low, and as the impurities are removed from the lithium nickel composite oxide, lithium is also extracted from inside the particles, and thus a decrease in the battery capacity and an increase in cathode resistance will occur due to lithium deficiency. Moreover, after the heat treatment, the amount of lithium on the surface decreases as compared to the amount of lithium inside the particles, so there is a possibility that a problem will occur in which the diffusion of lithium inside the cathode active material will become impaired, and that the conductive paths will become insufficient.